Optical discs such as the CD (Compact Disc: registered trademark), DVD (Digital Versatile Disc: registered trademark), or BD (Blu-ray Disc: registered trademark) are in wide use, ranging from industrial to consumer use, because they can provide relatively inexpensive high-capacity information recording media on which information can be recorded and reproduced without contact. Optical disc capacity can be increased by reducing the size of the recording marks (including pits and phase-change marks) formed in the track-like or spiral recording tracks on the optical disc; this has been achieved by shortening the wavelength of the laser beam used in recording and reproduction and increasing the numerical aperture (NA) of the objective lens, thereby reducing the size of the focused spot on the focal plane, in keeping with the reduced size of the recording marks.
For example, a CD can provide a storage capacity of 650 MB with a disc substrate functioning as a light transmitting layer substantially 1.2 mm thick, a laser beam wavelength of substantially 780 nm, and an objective lens with a 0.45 NA. A DVD can provide a storage capacity of 4.7 GB with a disc substrate functioning as a light transmitting layer substantially 0.6 mm thick, a laser beam wavelength of substantially 650 nm, and a 0.6 NA. The higher density BD can provide a large capacity of 25 GB or more with a thin (substantially 0.1 mm thick) protective layer functioning as a light transmitting layer covering the optical recording layer, a laser beam wavelength of substantially 405 nm, and a 0.85 NA.
The technology of shortening of the laser beam wavelength and increasing of the numerical aperture is reaching its limits, however, making it difficult to advance to greater capacities by the methods that have been used so far. The mainstream trend in development work has therefore become to achieve higher capacity by means of multiple layers, by providing a stacked plurality of recording layers, thereby increasing the amount of recorded information per unit area on the optical disc.
In this type of multiple-layer optical disc, the thickness of the light transmitting layer between the lens and the recording layer on which recording or reproduction takes place varies depending on the recording layer selected, so spherical aberration occurs, degrading recording and reproduction performance. Therefore, in the optical pickup device, a collimator lens is generally disposed between the laser light source and the objective lens, and spherical aberration is corrected by moving the collimator lens.
In the correction of spherical aberration by means of a collimator lens, the more variation there is in the thickness of the light transmitting layer, the more the collimator lens must move. This means that as the number of recording layers increases, the amount of collimator lens movement also increases. A problem is that to provide for the increasing amount of collimator lens movement, the optical pickup device becomes larger in size.
It is possible to reduce the amount of collimator lens movement and still obtain high spherical aberration correction performance by shortening the focal length of the moving collimator lens. In conventional optical pickups, the collimator lens function is implemented by a combination of two lenses, and the amount of movement of these two lenses is reduced (see, for example, Patent Reference 1).